Safety paper and ink therefor



phenylmethane dyes.

pecially advantageous since they will develop Patented July 10, 1945 UNITED SATES SAFETY PAPER AND INK THEREFOR No Drawing. Application May 19, 1942, Serial No. 443,64?!

8 Claims.

This invention relates to ink compositions for safety paper, to safety paper having warning indicia composed of such inks, and to methods of stabilizing the color-forming constituents of such inks against premature development of color prior to the application of ink eradicators.

Safety paper is that designation given to bank checks, drafts, and. other writings or printings provided with, or impregnated with, chemicals which develop color upon the application of ink eradicators. Such eradicators usually consist of combinations of hypochlorites with acids and, more recently, combinations of permanganates with bisulfltes. Various fraud-preventative inks for the. treatment of such papers are in use. Among the more successful is a class of chemical compounds of the benzidine series. Benzidine sulfate is a typical example thereof.

More recently I have discovered, and have separately claimed in my co-pending application, Serial No. 383,030, new issued as Patent No. 2,288,- 147, dated June 30, 1942, the use of colorless, or substantially colorless, leuco derivatives of tri- Such leuco dyes are escolor upon the application of permanganate eradicating agents, and such color is not reduced by the subsequent application of a bisulfite solution. Thus the leuco derivatives of the triphenylmethane dyes are color-forming to give permanent discoloration with both the hypochlorite eradicators and the permanganate-bismfite eradicators. Frequently it is advantageous to combine color-forming compounds of the benzidine series with the leuco derivatives of the triphenylmethane dyes, and such subject matter is described and claimed in the aforesaid application.

There are, however, relatively few commercially suitable color-forming substances for use in safety paper. Many compounds have been suggested but relatively few are in actual use.

Those compounds of the benzidine series enjoy wide popularity at the present time and are eminently successful for the purpose intended.-

In the ordinary application of fraud-preventative inks to safety paper for the printing of warning indicia, the ink is incorporated in a film-forming vehicle, such as a solution of ethyl cellulose in dimethyl phthalate. This ink vehicle tends to protect the color-forming constituent, such as benzidine sulfate, against atmospheric oxidation. However, in spite of the presence of a film in which the benzidine sulfate is incorporated, there have been occasional instances where warning indicia, such as the word void have spontaneously developed color. In the vast majority of instances these various color-forming constituents remain colorless until an ink eradicator is applied. But while the failures are relatively few in number, nevertheless, an

occasional discoloration will, as stated, develop without the application of an ink eradicator.

The reason for such failures has been very obscure. No one has hitherto understood the reason at all. In an endeavor to solve the problem presented by these occasional failures, I have discovered the reason why color-forming compounds of this type will occasionally develop color spontaneously. And I have, as a consequence of understanding the cause of the failure, been able to correct it and thus completely protect the safety paper against any deterioration or spontaneous development of color until deliberate attempts are made to alter writing on such safety paper by th application of ink eradicators. My invention is based upon the discovery that the reason why occasional coloration in such inks develops is because of certain hydrogen ion concentration relationships between the paper itself and the hydrogen ion concentration at which the color-forming compound tends to show instability and susceptibility to spontaneous coloration. So

far as I am aware, no one has hitherto appreciated this factor in connection with the manufac ture of safety paper.

My invention is based upon the fact that the color-forming compounds, such as the benzidines, and the triphenylmethane leucos, as well as others whose color formation is dependent in part upon hydrogen ion concentration, become unstable when the pH of the ink containing such compounds is increased. For example, colorless benzidine sulfate has a normal pH of approximately 2 to 2.5. If the pH is increased to 3.5, then the benzidine sulfate hydrolyzes to some slight extent and the hydrolyzed product may spontaneously develop color. This color formation may not occur immediately but gradually develops if the paper is kept for some period of time before use.

Since the color-forming constituent when first made and incorporated in the ink is at a pH rendering it stable, I still was obliged to discover the reason wh the pH in the ink after application thereof to the paper was changed. After many experiments, I learned that the pH value of the paper was about 4.5 to 5, and because the pH value of the paper was higher than. that of the color-forming compound, exposure to abnormal humidity or slight traces of ammonia or other fumes that may be due to perspiration when checks are carried on th person would gradually cause an increase in the pH of the ink on the paper whereby the color-forming compound therein slowly hydrolyzed and thus became susceptible to spontaneous color change.

All safety paper, I learned, has a very slight acid reaction, the pH being about 4.5 to 5, as stated. The color-forming constituents, however, have a more highly acid reaction since the pH of a stable, normal benzidine sulfate is about 2. However, when paper is impregnated or printed with the 'benzidine sulfate the paper, being more alkaline, that is to say, the paper being less acid than the dye, the pH of the dye tends to be increased, and at values as high as 3 or 3.5 hydrolysis of the benzidine salt sets in and the hydrolysis product is susceptible to spontaneous coloration.

After having discovered the relationship and effect between the pH of the paper and the PH at which the color-forming constituent is stable, I then was able to discover ways by which the pH of the color-forming constituent could be maintained throughout the life of the paper at those values which prevented any tendency to hydrolysis with resultant instability.

This problem, I found, could not be solved by increasing the acidity of the paper to the'point at which the dye applied thereto would always be stable. Paper having an acidity expressed as a pH of 2 is brittle and entirely unsuitable for use.

The problem was solved by incorporating in the ink acid-reacting substances which would always maintain the pH of the ink within those limits making for stability of the color-forming constituent. However, the simple addition of an acid, for example, to the ink was not satisfactory. While the DH of the ink could be kept well below 3 by the addition of an inorganic acid or even an inorganic acid salt, such substances tended to migrate into the paper and weaken the paper considerably in those areas where migration occurred.

I finally discovered a class of compounds which could be added to the ink to maintain the desired pH values therein without deleteriously affecting the paper in any way, and which would not materially migrate into the paper. These compounds are inert towards the dye. This class of compounds is very wide, and I do not wish to be limited to any specific members thereof. All of the substances I use for stabilizing can be generically described as acid-reacting organic esters of inorganic acids, "or as organic derivatives of such acids. Such compounds are well known in the art. The sulfonic acids, particularly benzene sulfonic acids, are well known. Paradiphenyl disulfonic acid is especially suitable for my purpose. Phosphoric acid esters of organic nature are illustrative of another class of compounds I can use. Glyceryl phosphoric acid is a well known member of this class. The sulfated fatty acids, alcohols, and the like, are other examples. Turkey red oil, a sulfated castor oil, is a specific member of this class. These specific compounds are all organic compounds containing an inorganic acid radical selected from a class consisting of the sulfo and phospho radicals. I am not here thinking in terms of wetting agents per so, although some of these sulfuric acid derivatives of organic compounds have the additional property of being wetting agents.

I do not use these compounds as wetting agents at all but as buffering agents tending to stabilize the pH of the ink.

Any of the acid-reacting organic derivatives of inorganic acids can be used as stabilizers for the color-forming constituents in the practice of the present invention. The amount thereof to be incorporated in the ink should be such that suiIL.

cient is present to nullify any effect of the lesser acid-reacting paper on the color-forming constituents in the ink. To put the matter another way, enough stabilizer is added to maintain the colorforming constituent at that pH at which no tendency to hydrolyze will develop.

With the foregoing in mind it will be apparent to those skilled in the art that; there are many different ways of practicing my invention.

For example, one suitable ink composition has the following formula: 40 parts by weight of benzidine sulfate and 30 parts by weight of paradiphenyl disulfonic acid are mixed together and incorporated in a suitable vehicle, such as a. 4% solution of ethyl cellulose in dimethyl phthalate. In the above formula, parts of such avehlcle are used. Obviously the amount of vehicle can be varied in accordance with the practice in this art. I lay no claim to any special vehicle. Instead of the disulfonic acid stabilizer referred to, I can use benzene sulfonic acid itself. The 2,5-dichlorbenzene sulfonic acid is equally as eiIective.

Instead of using aromatic sulfonic acids I can substitute Turkey red oil which is a sulfated fatty acid.

Another composition consists of 2 parts of glycerine, one part of glyceryl phosphoric acid as a stabilizer and 2 parts of the leuco of alpha azurine Color Index #714. This leuco is a leuco of a triphenylmethane dye.

The choice of acid-reacting stabilizer is governed in part upon the vehicle of the ink and in part upon the color-forming constituent to be stabilized. It is desirable, of course, to use organic acid-reacting stabilizers which are soluble in and compatible with the vehicle in order to localize their action in the vehicle and thus keep them in contact with the color-forming compound.

Indeed, the ink vehicle itself can be a sulfated or sulfonated organic derivative, since many of these derivatives are solvents for the colorforming compound. A striking and effectual illustration is a solution of ethyl cellulose in the sulfated oils or the mono and di phosphoric acid ester of propyl, butyl or octyl alcohols. Varnish or the ink medium made of these products have no tendency to migrate into the paper and are exceedingly eifectual in surrounding the particles of color-producin material with a highly acid envelope that preserves them from premature development.

The ink is, of course, applied to the safety paper in ways customarily used in this art. For example, warning indicia, such as the Word "void to be printed with such inks, and when an eradicator is applied the word flashes up in color. But because of the presence of the stabilizer in the ink no premature development of color occurs even though the paper he kept for long periods before use.

Although I have more specifically referred to the benzidine type, and leuco triphenylmethane type of color-forming agent, I have used these compounds primarily as illustrative and not as limiting. These compounds are of particular utility in this art. Others, however, which are stable at a particular pH value and less so at a higher pH value, can be stabilized in accordance with the teachings of my invention.

I have particularly referred to sulfated and sulfonated organic acid-reacting compounds, and to various phosphoric acid esters since these are generally available for use. But it will be apparent to those skilled in the art that my invention can be practiced with other acid-reacting organic derivatives to maintain the pH of the colorforming constituent at that which inhibits rapid or gradual spontaneous color development.

4 In the appended claims I use the word sulfo" to generically define the sulfated and sulfonateo organic acid-reacting compounds I can use. Likewise, phospho is a convenient generic expression embracing the various acid-reacting organic compounds of phosphorus useful in my invention.

Having thus described my invention, what I claim is:

1. A safety paper bearing an alteration-indicating composition comprising a film-forming vehicle, a normally-colorless dye which develops color when subjected to the action of ink eradi- "cators for indicating attempted alteration thereof and tending to develop color prematurely due to hydrolysis upon increase in pH when in contact with the paper and an acid-reacting nonvolatile bufiering agent which is an organic compound containing an inorganic acid radical selected from a class consisting of the phospho and sulfo radicals, soluble in and compatible with said vehicle when in the form of a dry film and adapted to stabilize the pH of said composition at a value below that of the paper and below the point producing hydrolysis of the dye and inert towards the dye.

2. A safety paper bearing an alteration-ind1- eating composition comprising a film-forming vehicle, a normally-colorless dye which develops color when subjected to the action of ink eradicators for indicating attempted alteration thereof and tending to develop color prematurely due to hydrolysis upon increase in pH when in contact with the paper which has a pH suficiently high to produce hydrolysis of said dye, and a non-volatile acid buffering agent, which is an acid-reacting organic ester of an inorganic acid, soluble in and compatible with said vehicle when in the form of a dry film and adapted to stabilize the pH of said composition at a value be-= low that of the paper and below the point producing hydrolysis of the dye and inert towards the dye.

3, A safety paper bearing an alteration-indicating composition comprising a film-forming vehicle, a normally-colorless dye which develops color when subjected to the action of ink eradicators for indicating attempted alteration thereor and tending to develop color prematurely due to hydrolysis upon increase in pH when in con tact with the paper which has a pH sufiiciently high to produce hydrolysis of said dye, and a non-volatile acid buffering agent, which is an acid-reacting organic ester of an inorganic acid selected from a class consisting of the acidreacting organic sulfo and phospho compounds, soluble in and compatible with said vehicle when in the form of a dry film and adapted to stabilize the pH of said composition at a value below that of the paper and below the point producing hydrolysis of the dye and inert towards the dye.

4. The safety paper of claim 2 wherein the dye comprises a benzidine compound.

5. An alteration-indicating composition for application to safety paper comprising a filmforming vehicle, a normally-colorless dye which develops color when subjected to the action of ink eradicators and which tends to develop color prematurely due to hydrolysis upon increase in pH when in contact with safety paper, and an acid-reacting buffering agent, which is an organic compound containing an inorganic acid radical selected from the class of sulfo and phospho radicals, soluble in and compatible with said vehicle when in the form of a dry film and adapted to stabilize the pH of said composition at a value below that of the paper and below the point producing hydrolysis of the dye and inert towards the dye.

6. An alteration-indicating composition for application to safety paper comprising a filmforming vehicle, a normally-colorless dye which develops color when subjected to the action of ink eradicators and which tends to develop color prematurely due to hydrolysis upon increase in pH when in contact with safety paper, and an acid bufiering agent, which is a non-volatile acid-reacting organic ester of an inorganic acid, soluble in and compatible with said vehicle when in the form of a dry film and adapted to stabilize the pH of said composition at a value below that of the paper and below the point producing hydrolysis of the dye and inert towards the dye.

Z. An alteration-indicating composition for application to safety paper comprising 'a filmforming vehicle, a normally-colorless dye which develops color when subjected to the action of inkeradicators and which tends to develop color prematurely due to hydrolysis upon increase in pH when in contact with safety paper, and a nonvolatile acid buffering agent, selected from a class consisting of the acid-reacting organic sulfo and phospho esters, which is an acid-reacting organic ester of an inorganic -acid, soluble in and cornpatible with said vehicle when in the form of a dry film and adapted to stabilize the pH of said composition at a value below that of the paper and below the point producing hydrolysis of the dye and inert towards the dye.

8. The composition of claim 6 wherein the dye comprises a benzidine compound.

BURGESS W. SMITH. 

